1. Technical Field
The present invention relates to an ignition coil for use in an internal combustion engine which is connected directly to an ignition plug and is accommodated in a plug hole of the internal combustion engine and further relates an internal combustion engine having such an ignition coil.
2. Prior Art
In a conventional ignition coil, as shown in Japanese patent laid-open publication No. Hei 3-149805, as a potting type resin for a fixing and an insulation of the components such as a primary coil and a secondary coil etc., a thermoplastic resin comprised of an epoxy resin is used and further as a filling-up agent a crystalline silica is mainly employed.
In the conventional type ignition coil, since the ignition coil is accommodated in a plug hole having a diameter of 20-30 mm degree, a size in a radial direction of the coil is small and a distance between the primary coil and an outer side magnetic core is short. As a result, there is a concern that an insulation between the secondary coil having a high potential, the primary coil having a substantial ground potential and the outer side magnetic core can not be achieved. Particularly, since a coil interior portion is exposed a high electric field under a high temperature of more than 150.degree. C., there is a problem of inconvenience about an insulation endurance performance causes in a comparative short time.
As a technique to cope with the above problem, a known technique (namely, an inside secondary coil method is used), in which the primary coil having the substantial ground potential is arranged at an outer side-magnetic core side and the secondary coil having the high potential is arranged at a center magnetic core side.
In this conventional technique, the center magnetic core presents a floating potential and a potential difference causes between the secondary coil having the high potential and the center magnetic core but since the potential difference is about half that of an outer side secondary coil system of an ignition coil in which a secondary coil is arranged at an outer side magnetic core, accordingly it has a merit that an insulation performance is superior.
However, as to the above stated inside secondary coil system ignition coil, in an operation endurance test in a heat cycle oven in which a drive mode of an actual vehicle is supposed, a breakdown between a secondary winding and an outer side magnetic core causes according to a heat shock, therefore there is a problem of inconvenience (a disturbance in a rotation of the engine) in the internal combustion engine. As a result of the study of this cause, it has become clear that since a thermal stress is concentrated at a vicinity of a crossover wire portion of a winding finish portion of the secondary coil, minute voids generated in an insulation resin which is filled up at a neighborhood of the above stated winding crossover wire portion vicinity and an electric treeing generates at this portion according to the voids as a trigger and this electric treeing in a destruction of insulation in the ignition coil occurs.